Conformations of Alkanes
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Conformations are different forms of a molecule, related by simple rotation about a single bond. Such interconversions are usually very rapid, so that a sample of a given molecule may exist in many different conformations.
For alkanes, various conformations can be represented by using dash-wedge notation in a structural "perspective" drawing. An alternate depiction is a Newman Projection, which views a molecule by looking down a C-C bond axis, showing the relative orientation of groups off the two carbon atoms.
This conformation of ethane is the staggered form. The various hydrogen atoms on the two carbons form a dihedral angle of 60º.
This conformation of ethane is the eclipsed form. The hydrogen atoms have a dihedral angle of 0º.
The staggered conformation of ethane is more stable than the eclipsed form by 12.1 kJ/mol, so that as one methyl group rotates 360º relative to the other, the compound passes through three stable staggered conformers via three unstable eclipsed forms. This barrier is small enough that at 25ºC the compound changes conformation about 50 million times each second!
Butane has not only eclipsed and staggered conformations, but also forms that vary in the relative orientation of the methyl groups.
|+15.9 kJ/mol||+3.8 kJ/mol||+18.8 kJ/mol|
|As with ethane, the eclipsed conformations are higher energy than the staggered. The staggered conformation where the two methyl groups are as far away from one another as possible (with a dihedral angle of 180º, seen in the Newman projection) is called the anti conformation, and is the lowest-energy arrangement. The two staggered forms with the methyl groups in closer proximity (60º) are the gauche conformations. Direct interconversion of one gauche form to the other requires passing through the highest-energy eclipsed form, where the two methyl groups are next to each other. A space-filling representation of this conformation shows that the methyl groups are physically touching, leading to the high energy.|
This page is maintained by W. Stephen McNeil.